Method for determining a state of a cell of a battery

ABSTRACT

A method for determining a respective state of each of a plurality of cells of a battery, comprising at least the following steps: a) carrying out a charging operation or a discharging operation on the cells; b) determining a discharge voltage of each of the cells and a charge voltage of each of the cells; c) determining at least one state parameter for each cell, wherein the state parameter is derived from the discharge voltage and the charge voltage, wherein a discharge voltage and a charge voltage of at least one other cell is taken into account for the state parameter.

The invention relates to a method for determining the state of a cell ina battery, particularly of a high-voltage battery. In particular, themethod is aimed at determining a state of a cell while taking the statesof other cells of the same battery into account.

Such high-voltage batteries are used particularly in motor vehicles forthe purpose of storing electric power for driving traction drives. Abattery is normally composed of a plurality of cells, with each cellhaving a terminal voltage of 1.5 to 4 volts. The cells are at leastpartially connected in series, whereby a traction voltage of 60 to 1,500volts of direct current is provided.

In light of current and future legislation, it is necessary to detectthe state of the battery or of the cells when a motor vehicle is inoperation. In particular, by determining the state of the cells, earlyfailure detection and early service detection can be performed, therebyavoiding failure of a cell or battery during operation and enabling theprompt replacement of the cell or battery.

The known methods for determining the state of a cell or a batteryusually only provide snapshots, with the effect that the evolution ofthe state (e.g., SOH—state of health) of the cell or battery is notdetected.

A method for determining a state of health of a battery cell is knownfrom DE 10 2009 000 337 A1. An impedance spectrum of the battery cell isrecorded.

A method for monitoring a lithium-ion battery cell is known from DE 102011 117 249 A1. There, a load capacity is differentiated according tothe corresponding battery cell voltage.

A method for operating a secondary battery is known from DE 10 2014 214314 A1. The state parameters of individual cells are detected, and anoperating strategy is derived from them.

It is the object of the present invention to at least partially solvethe problems described herein with reference to the prior art. Inparticular, a method for determining a state of a cell of a battery isto be provided. In particular, the method should make it possible toobtain information on the evolution of the state over a service life.

A method with the features according to claim 1 contributes to theachievement of these objects. Advantageous developments are the subjectof the dependent claims. The features listed individually in the claimscan be combined in a technologically meaningful manner and supplementedby explanatory facts from the description and/or details of the figures,with additional design variants of the invention being indicated.

A method for determining a respective state of a plurality of cells of abattery is proposed. In particular, the method should be able todetermine the state of at least one cell of the battery, preferably ofevery cell of the battery.

The method comprises at least the following steps:

-   -   a) carrying out a charging operation or a discharging operation        on the cells;    -   b) determining a discharge voltage of each of the cells and a        charge voltage of each of the cells;    -   c) determining at least one state parameter for each cell, the        state parameter being derived from the discharge voltage and the        charge voltage, with a discharge voltage and a charge voltage of        at least one other cell being taken into account for the state        parameter.

The above (non-exhaustive) breakdown of the method steps into a) throughc) is primarily intended to serve purposes of distinction and not toimpose any order and/or dependency.

The frequency of the method steps, e.g., during the setting-up and/oroperation of the system, may vary. It is also possible for method stepsto overlap temporally at least in part. Method steps b) and c) veryespecially preferably take place during or immediately after step a). Inparticular, steps a) to c) are carried out in the order listed.

The charging operation and discharging operation each refer to acharging process in which the cell is exclusively supplied with anelectric current (charging operation) or an electric current isexclusively removed from the cell (discharging operation). Inparticular, each charging operation can be evaluated using the method,regardless of the amount of electrical current supplied or removed.

As part of step b), a discharge voltage and a charge voltage aredetermined or measured for each of the cells under consideration. Thedischarge voltage describes the voltage of the cell after thedischarging operation. The charge voltage describes the voltage of thecell after the charging operation. In particular, the voltages of theplurality of cells are determined at the same point in time, i.e., alldischarge voltages at a common point in time and all charge voltages atanother common point in time.

The discharge voltage is particularly the lowest voltage of a chargingoperation. The charge voltage is particularly the highest voltage of acharging operation.

In particular, a charging operation can be used to carry out the methodmultiple times. For example, the respective voltage of the cells can bedetermined at specific points in time during the ongoing chargingoperation, and the method can be carried out taking these voltages intoaccount.

As part of step c), at least one state parameter is determined for eachcell under consideration. This is derived from the discharge voltage andthe charge voltage of this cell. However, the corresponding voltages ofat least one other cell, but particularly of all other cells underconsideration, are also taken into account.

In particular, the state parameter is determined by normalization. Thenormalization enables the states of the different cells to be comparedwith one another.

In particular, the at least one state parameter is determined for atleast a plurality of charging operations, with an evolution of the stateparameters determined in this manner being taken into account.

Since the method can be carried out at any time and for any type ofcharging operation and independently of the amount of electrical currentremoved or supplied, a plausibility check of previous results can beeasily implemented. This enables previously determined state parametersto be checked or verified by frequent repetition of the method. Inaddition, an evolution of the state parameters and thus of the state ofthe cell can be tracked with high temporal resolution.

In particular, one state parameter is at least one capacity parameter orequilibrium parameter. The capacity parameter describes a ratio of thedischarge voltage and the charge voltage of a cell taking the ratio ofother cells into account. In particular, the capacity parameter thusdescribes the difference between the discharge voltage and the chargevoltage of a charging operation of a cell. A large differencecorresponds to a small capacity of the cell, since a small amount ofelectric current brings about a large difference in the voltage of thecell. Conversely, a small difference corresponds to a high capacity ofthe cell.

The equilibrium parameter describes a discharge voltage level and acharge voltage level of a cell compared to the respective voltage levelsof other cells. In particular, the equilibrium parameter describes thedifference between a first charge voltage level, or value, of a cellcompared to the respective first charge voltage level of the other cellsand a second discharge voltage level of the cell compared to therespective second discharge voltage level of the other cells.

A negative equilibrium, for example, means that a cell that is mostdeeply discharged compared to the other cells, i.e., the one having thelowest discharge voltage among all of the cells, is charged the least ina charging operation—i.e., it has the lowest charge voltage among all ofthe cells.

A positive equilibrium, for example, means that a cell that is leastdeeply discharged compared to the other cells, i.e., the one having thehighest discharge voltage among all of the cells, is charged the most ina charging operation—i.e., it has the highest charge voltage among allof the cells.

A balanced equilibrium, for example, means that a cell that is thethird-least discharged compared to the other cells, i.e., the one havingthe third-lowest discharge voltage among all of the cells, is chargedthe third-most in a charging operation—i.e., it has the third-highestcharge voltage among all of the cells.

In particular, the discharge voltage and charge voltage determined foreach cell is normalized in order to determine the state parameters. Thefollowing applies to the normalized charge voltage x_(i) of a cell i:

$x_{i} = \frac{U_{xi} - U_{xmin}}{U_{xmax} - U_{xmin}}$

where the following applies for the normalized discharge voltage y_(i)of a cell i:

$y_{i} = {\frac{U_{yi} - U_{ymin}}{U_{ymax} - U_{ymin}} - {1.}}$

Here,

U_(xi): is the charge voltage of the cell i under consideration,

U_(xmin): is the maximum charge voltage of all of the cells i=1 to nunder consideration,

U_(xmin): is the minimum charge voltage of all of the cells i=1 to nunder consideration,

U_(yi): is the discharge voltage of the cell i under consideration,

U_(ymin): is the maximum discharge voltage of all of the cells i=1 to nunder consideration,

U_(xmin): is the minimum discharge voltage of all of the cells i=1 to nunder consideration,

The number of cells here is n, where i or n is a natural number, i.e.,n=2, 3, 4, . . .

The normalization for the charge voltage is therefore based on adifference between the maximum charge voltage among all of the cells andthe minimum charge voltage among all of the cells in this chargingoperation.

The normalization for the discharge voltage is therefore based on adifference between the maximum discharge voltage among all of the cellsand the minimum discharge voltage among all of the cells in thischarging operation.

The normalization enables one cell to be compared with the other cellsin the battery.

In particular, the at least one state parameter is a capacity parameterC_(i) of a cell i, where: C_(i)=2−(x_(i)−y_(i)).

In particular, the at least one state parameter is an equilibriumparameter B_(i) of a cell i, where: B_(i)=1−|x_(i)+y_(i)|.

In particular, a reciprocal of the state parameter is considered inorder to determine the state of a cell. The reciprocal of C_(i) isparticularly 1/C_(i). The reciprocal of B_(i) is particularly 1/B_(i).

In particular, an intervention limit is defined for each stateparameter, and when this is exceeded, a repair status is decided uponfor the relevant cell.

In particular, the intervention limit is determined as a function of thestate parameters determined for the majority of the cells.

In particular, an arithmetic mean of these state parameters of the cellsunder consideration is formed for the respective state parameter, withthe intervention limit being at least 130%, preferably at least 150%, oreven 200% of this mean value, for example.

In particular, the intervention limit for the last charging operation ordischarging operation is redetermined. In particular, the arithmeticmean can be recalculated for each charging operation, for example. Thisenables continuous degradation of the cells over their lifetime to betaken into account in particular.

Particularly, the method can be implemented in a control unit, thecontrol unit being provided at least for diagnosing and optionally alsofor operating the battery.

The battery can be used in a motor vehicle to store energy, with atleast one traction drive of the motor vehicle being supplied withelectric power via the battery.

What is proposed in particular is a motor vehicle with a traction driveand the described battery assembly.

A control device or a data processing system is also proposed which isequipped, configured, or programmed to carry out the method described.

Furthermore, the method can also be carried out by a computer or with aprocessor of a control unit or of a data processing system.

Accordingly, a system for data processing is also proposed whichcomprises a processor that is adapted/configured in such a way that itcarries out the method or a portion of the steps of the proposed method.In particular, the system for data processing for determining the stateof a plurality of cells of a battery comprises at least one voltagedetector for determining or measuring the voltage (e.g., charge voltageand discharge voltage) and means suitable for carrying out the steps ofthe method described.

A computer-readable storage medium can be provided that comprisesinstructions which, when executed by a computer/processor, cause thelatter to carry out the method or at least a portion of the steps of theproposed method.

Remarks concerning the method can be applied particularly to the batteryassembly, the motor vehicle, and/or the computer-implemented method(i.e., the computer or processor, data processing system,computer-readable storage medium), and vice versa.

Particularly in the claims and in the description that describes them,the indefinite articles (“a” and “an”) are to be understood as such andnot as quantifiers. Accordingly, terms and components that areintroduced therewith are thus to be understood as being present at leastsingly but particularly also possibly in a plurality.

By way of precaution, it should be noted that the number words used here(“first,” “second,” . . . ) serve primarily (only) to distinguish aplurality of similar objects, quantities, or processes; that is, they donot prescribe any dependency and/or order of these objects, quantities,or processes relative to one another. Should a dependency and/or orderbe required, this is explicitly stated herein or it obviously followsfor a person skilled in the art when studying the embodimentspecifically described. If a component can occur multiple times (“atleast one”), the description of one of these components can applyequally to all or a portion of the plurality of these components, butthis is not necessarily the case.

The invention and the technical environment will be explained in greaterdetail with reference to the enclosed figures. It should be noted thatthe invention is not intended to be limited by the specifiedembodiments. In particular, unless explicitly stated otherwise, it isalso possible to extract partial aspects of the features explained inthe figures and to combine them with other components and insights fromthe present description. In particular, it should be pointed out thatthe figures and, in particular, the illustrated proportions are onlyschematic. In the drawings:

FIG. 1 shows two diagrams illustrating a charging operation of abattery;

FIG. 2 shows three diagrams illustrating an interim dischargingoperation;

FIG. 3 shows a diagram of a cell with a low capacity;

FIG. 4 shows a diagram of cell with a high capacity;

FIG. 5 shows a diagram of a cell having a negative equilibrium;

FIG. 6 shows a diagram of a cell having a positive equilibrium;

FIG. 7 shows a diagram of a cell having a balanced equilibrium;

FIG. 8 shows two diagrams of a charging operation of a plurality ofcells and the nominalized voltages thereof;

FIG. 9 shows a diagram of the capacity parameters of the plurality ofcells of FIG. 8 ;

FIG. 10 shows a diagram of the equilibrium parameters of the pluralityof cells of FIG. 8 ;

FIG. 11 shows a diagram of the reciprocals of the capacity parametersaccording to FIG. 9 ;

FIG. 12 shows a diagram of the reciprocals of the equilibrium parametersaccording to FIG. 10 ;

FIG. 13 shows three diagrams of the normalized voltages, the reciprocalsof the capacity parameters, and the reciprocals of the equilibriumparameters of all of the cells for a charging operation; and

FIG. 14 shows three graphs of the reciprocals of the capacity parametersof all of the cells for three different charges.

FIG. 1 shows two diagrams of a charging operation 6 of a battery 5. Thevoltage 12 is plotted on the vertical axes of the diagrams. The time 13is plotted on the horizontal axis of the diagrams. The battery 5comprises a plurality of cells 1, 2, 3, 4. At the beginning of thecharging operation 6, discharge voltages 8 of each of the cells 1, 2, 3,4, . . . n are measured, where n=88. At the end of the chargingoperation, the charge voltages 9 of each of the cells 1, 2, 3, 4, . . .n are measured.

FIG. 2 shows three diagrams of an interim discharging operation 7. Thistakes place during the charging operation 6 according to FIG. 1 . Thevoltage 12 is plotted on the vertical axes of the diagrams, and thecurrent 14—here the discharge current—is plotted on the upper rightdiagram. The time 13 is plotted on the horizontal axis of the diagrams.It can be seen that the cells 1, 2, 3, 4 have different voltage profiles12 while the battery 5 is being discharged with the current 14.Reference is made to the remarks in relation to FIG. 1 .

The following FIGS. 3 to 7 each show profiles for the voltages 12 ofindividual cells, the battery 5 examined there having a total of 88cells, here from cell 0 to cell 87. The respective profile of a voltagedescribed below is always explained as an example for a cell which isreferred to here as the first cell 1.

FIG. 3 is a diagram showing a cell 1 with a small capacity. FIG. 4 is adiagram showing a cell 1 with a large capacity. FIGS. 3 and 4 aredescribed together below.

The voltage 12 is plotted on the vertical axes of the diagrams. The time13 is plotted on the horizontal axis of the diagrams. The identifiablygreat difference between the discharge voltage 8 and the charge voltage9 of the first cell 1 visible in FIG. 3 corresponds to a low capacity ofthe cell 1, since a small amount of electric current 14 brings about agreat difference in the voltage 12 of the cell 1. Conversely, a smalldifference corresponds to a high capacity of the cell 1. This situationis illustrated in FIG. 4 .

FIG. 5 shows a diagram of a cell 1 having a negative equilibrium. FIG. 6shows a diagram of a cell 1 having a positive equilibrium. FIG. 7 showsa diagram of a cell 1 having a balanced equilibrium. FIGS. 5 to 7 aredescribed together below.

The voltage 12 is plotted on the vertical axes of the diagrams. The time13 is plotted on the horizontal axes of the diagrams.

A negative equilibrium, which is illustrated in FIG. 5 , means that afirst cell 1 that is most deeply discharged compared to the other cells2, 3, 4, i.e., the one having the lowest discharge voltage 8 among allof the cells 1, 2, 3, 4, is charged the least in a charging operation6—i.e., it has the lowest charge voltage 9 among all of the cells 1, 2,3, 4.

A positive equilibrium, which is illustrated in FIG. 6 , means that afirst cell 1 that is least deeply discharged compared to the other cells2, 3, 4, i.e., the one having the highest discharge voltage 8 among allof the cells 1, 2, 3, 4, is charged the most in a charging operation6—i.e., it has the highest charge voltage 9 among all of the cells 1, 2,3, 4.

A balanced equilibrium, which is illustrated in FIG. 7 , means that afirst cell 1 that is the third-least discharged compared to the othercells 1, 2, 3, 4, i.e., the one having the third-lowest dischargevoltage among all of the cells 1, 2, 3, 4, is charged the third-most ina charging operation 6—i.e., it has the third-highest charge voltage 9among all of the cells 1, 2, 3, 4.

FIG. 8 shows two diagrams illustrating a charging operation 6 of aplurality of cells 1, 2, 3, 4 and the nominalized voltages 15, 16thereof. In the diagram on the left, the voltage 12 is plotted on thevertical axis. The time 13 is plotted on the horizontal axis. Thebattery 5 comprises a plurality of cells 1, 2, 3, 4. At the beginning ofthe charging operation 6, discharge voltages 8 of each of the cells 1,2, 3, 4 are measured. For example, the first cell 1 has a dischargevoltage 8 of 3.002 volts. At the end of the charging operation 6, thecharge voltages 9 of each of the cells 1, 2, 3, 4 are measured. Here,the first cell 1 has a charge voltage 9 of 4.131, for example.

In the diagram on the right, the value of the normalized charge voltage15 xi is plotted on the vertical axis above the horizontal axis, and thevalue of the normalized discharge voltage y_(i) 16 is plotted below thehorizontal axis. The cells 1, 2, 3, 4, i.e., n, are plotted on thehorizontal axis.

According

${{{to}x_{i}} = \frac{U_{xi} - U_{xmin}}{U_{xmax} - U_{xmin}}},$

the normalized charge voltage 15 of the first cell 1 is 1.0 here,whereas the charge voltage 9 of the first cell 1 under consideration hasthe value 4.131, the maximum charge voltage 9 among all of the cells 1,2, 3, 4 under consideration has the value 4.131, and the minimum chargevoltage 9 among all of the cells 1, 2, 3, 4 under consideration has thevalue 4.121.

According to

${y_{i} = {\frac{U_{yi} - U_{ymin}}{U_{ymax} - U_{ymin}} - 1}},$

the normalized discharge voltage 16 of the first cell 1 is −0.875 here,whereas the discharge voltage 8 of the first cell 1 under considerationhas the value 3.002, the maximum discharge voltage 8 among all of thecells 1, 2, 3, 4 under consideration has the value 3.009, and theminimum discharge voltage 8 among all of the cells 1, 2, 3, 4 underconsideration has the value 3.001.

FIG. 9 shows a diagram of the capacity parameters 10 of the plurality ofcells 1, 2, 3, 4 according to FIG. 8 . The capacity parameter 10 isplotted on the vertical axis. The cells 1, 2, 3, 4, i.e., n, are plottedon the horizontal axis.

The following applies to the capacity parameter C_(i) 10 of a cell i:C_(i)=2−(x_(i)−y_(i)). The capacity parameter 10 for the first cell 1 isthus 0.125 here.

FIG. 10 shows a diagram of the equilibrium parameters 11 of theplurality of cells 1, 2, 3, 4 according to FIG. 8 . The equilibriumparameter 11 is plotted on the vertical axis. The cells 1, 2, 3, 4,i.e., n, are plotted on the horizontal axis.

The following applies to the equilibrium parameter B_(i) 11 of a cell i:B_(i)=1−|x_(i)+y_(i)|. The equilibrium parameter 11 for the first cell 1is thus 0.85 here.

FIG. 11 shows a diagram of the reciprocals of the capacity parameters 10according to FIG. 9 . The reciprocal of the capacity parameter 10 isplotted on the vertical axis. The cells 1, 2, 3, 4, i.e., n, are plottedon the horizontal axis. The reciprocal of the capacity parameter 10 forthe first cell 1 is thus 8.0 here.

FIG. 12 shows a diagram of the reciprocals of the equilibrium parameters11 according to FIG. 10 . The reciprocal of the equilibrium parameter 11is plotted on the vertical axis. The cells 1, 2, 3, 4, i.e., n, areplotted on the horizontal axis. The reciprocal of the equilibriumparameter 11 for the first cell 1 is thus 1.14 here.

FIG. 13 shows three diagrams of the normalized voltages 15, 16, thereciprocals of the capacity parameters 10, and the reciprocals of theequilibrium parameters 11 for all of the cells 1, 2, 3, 4, . . . n,where n=88, for a charging operation 6. In the uppermost diagram, thevalues of the normalized charge voltage 15 xi are plotted on thevertical axis above the horizontal axis, and the values of thenormalized discharge voltage y_(i) 16 are plotted below the horizontalaxis.

In the center diagram, the reciprocals of the capacity parameter 10 areplotted on the vertical axis.

The intervention limit 17 for the reciprocal of the capacity parameter10 is defined as 2.0.

In the lower diagram, the reciprocals of the equilibrium parameter 11are plotted on the vertical axis.

The intervention limit 17 for the reciprocal of the equilibriumparameter 11 is defined as 3.0.

The cells that exceed the intervention limit 17 defined for therespective parameter (here the 7th, 8th, and 21st cell in the middlediagram as well as the 11th, 33rd, 50th, 52nd, 61st to 64th and 66thcell in the bottom diagram) can be identified. If these cells alsoexhibit corresponding abnormalities in the further course of the method,or if further deterioration occurs, these cells can be replaced in atargeted manner as necessary. A service date can be set according to thetrend of the change in the detected state parameters, so that a failureof the cell during operation is avoided, but an early replacement of thecell can also be prevented.

FIG. 14 shows three diagrams of the reciprocals of the capacityparameters 10 among all of of the cells 1, 2, 3, 4, . . . n, where n=88,for three different charging operations 6; see date information at thetop right of each diagram, i.e., September 7, September 10, andSeptember 11 of the same year.

In the diagrams, the reciprocals of the capacity parameter 10 areplotted on the vertical axis. The cells 1, 2, 3, 4, . . . n, where n=88,are plotted on the horizontal axis.

Cells have been identified here as possibly defective: in the topdiagram, the 7th, 8th, and 21st cell; in the middle diagram, the 7th andthe 65th cell; and in the bottom diagram, the 7th and the 26th cell.

It can be seen that the repeated measurements can also be used to carryout plausibility checks. Here, it can be seen that only the 7th cell islikely actually defective.

LIST OF REFERENCE SYMBOLS

1 first cell

2 second cell

3 third cell

4 fourth cell

5 battery

6 charging operation

7 discharging operation

8 discharge voltage

9 charge voltage

10 capacity parameter

11 equilibrium parameter

12 voltage

13 time

14 current

15 normalized charge voltage x_(i)

16 normalized discharge voltage y_(i)

17 intervention limit

1. A method for determining a respective state of each of a plurality ofcells of a battery, comprising at least the following steps: a) carryingout a charging operation or a discharging operation on the plurality ofcells; b) determining a discharge voltage of each of the plurality ofcells and a charge voltage of each of the plurality of cells; c)determining at least one state parameter for each of the plurality ofcells cell, wherein the state parameter is derived from the dischargevoltage and the charge voltage, and wherein a discharge voltage and acharge voltage of at least one other cell is taken into account for thestate parameter.
 2. The method as set forth in claim 1, wherein the atleast one state parameter is determined for at least a plurality ofcharging operations or discharging operations, and wherein an evolutionof the state parameters determined in this manner is taken into account.3. The method as set forth in claim 1, wherein one state parameter is atleast one capacity parameter or equilibrium parameter, wherein thecapacity parameter describes a ratio of the discharge voltage and thecharge voltage of a cell taking the ratio of other cells into account,and wherein the equilibrium parameter describes a discharge voltagelevel and a charge voltage level of a cell compared to the respectivevoltage levels of other cells.
 4. The method as set forth in claim 1,wherein the discharge voltage and charge voltage determined for eachcell is normalized in order to determine the state parameters, where thefollowing applies for the normalized charge voltage x_(i) of a cell i:${x_{i} = \frac{U_{xi} - U_{xmin}}{U_{xmax} - U_{xmin}}},$ where thefollowing applies for the normalized discharge voltage y_(i) of a celli: ${y_{i} = {\frac{U_{yi} - U_{ymin}}{U_{ymax} - U_{ymin}} - 1}},$ andwhere: U_(xi): is the charge voltage of the cell i under consideration,U_(sman): is the maximum charge voltage among all of the cells i=1 to nunder consideration, U_(xmin): is the minimum charge voltage among allof the cells i=1 to n under consideration, U_(yi): is the dischargevoltage of the cell i under consideration, U_(ymin): is the maximumdischarge voltage of all of the cells i=1 to n under consideration, andU_(xmin): is the minimum discharge voltage among all of the cells i=1 ton under consideration.
 5. The method as set forth in claim 4, whereinthe at least one state parameter is a capacity parameter C_(i) of a celli, and where the following applies: C_(i)=2−(x_(i)−y_(i)).
 6. The methodas set forth in claim 4, wherein the at least one state parameter is anequilibrium parameter B_(i) of a cell i, and where the followingapplies: B_(i)=1−|x_(i)+y_(i)|.
 7. The method as set forth in 6 claim 5,wherein a reciprocal of the state parameter is considered in order todetermine the state of a cell.
 8. The method as set forth in claim 7,wherein an intervention limit is defined for each state parameter, andwhen this is exceeded, a repair status is decided upon for the relevantcell.
 9. The method as set forth in claim 8, wherein the interventionlimit is determined as a function of the state parameters determined forthe plurality of cells.
 10. The method as set forth in claim 9, whereinthe intervention limit is newly determined for the last chargingoperation or discharging operation.